1. Field of the Invention
The present invention relates to a video signal recording/reproducing apparatus for recording video signals by bit rate reduction to allow the stabilized reproduction of video images.
2. Description of the Prior Art
In tune with the trend to adopt digital systems to various audio and video apparatuses and transmission channels, studies have been made on bit rate reduction systems for efficiently reducing data, and in video tape recorders (hereinafter referred to as VCRs) studies have been made with respect to methods of increasing the utilization efficiency of the tape by reducing the amount of data to be recorded, utilizing this technique. In general, bit rate reduction reduces the amount of data by deleting the two-dimension spatial superfluity and inter-frame superfluity by utilizing the correlations of video images. It extracts the characteristic of the video image and provides coding with respect to its characteristic. Furthermore, since the video image has individual characteristics for different parts of the pixel image, and since the amounts of data representing the respective characteristics are respectively different, variable length coding which provides different coding amounts according to the characteristics of the video image in a narrow range can provide a higher compression rate without deteriorating the encoded image quality as compared to fixed length coding for coding a certain amount of data in a certain narrow range of images.
The bit rate reduction allows curtailment of the amount of data in the information source with a high efficiency, but generates various problems in recording and reproducing the data of bit rate reduction on a VCR. For example, in a VCR, the signal to noise ratio (hereinafter referred to as the SNR) in reproducing is inferior to that in transmission and additionally dropout occurs. Therefore, even in carrying out recording and reproducing by adding an error correcting code, there may be cases where the data after the error correction includes bit errors. If the data includes errors, erroneous video signals will be decoded; in which case, in comparison with the case of recording the video signals without compression, when recording and reproducing are performed by bit rate reduction, the image quality is deteriorated over a wide range on a decoded pixel having one bit error. Moreover, when variable length bit rate reduction is performed, errors are propagated to other data, causing the deterioration of the image quality over a wider range on the decoded pixel.
In a VCR, search is essential, and the reproducing head crosses the video track. It is difficult to decode the search image from the reproduce variable length data. Further, s the correction capacity in searching generally decreases drastically in comparison with the case of normal reproducing, bit errors are increased.
A method of converting the video signal using a variable length bit rate reduction, recording it on a VCR and decoding the discriminating search image, is disclosed in Japanese Patent Publication (Unexamined) No. 63-95791/1988. Japanese Patent Publication (Unexamined) No. 63-95791/1988 relates to a coding apparatus in which a two dimension region formed by a video signal is divided into plural blocks, each of which is subjected to variable length coding to obtain coded data of a fixed length and a variable length; the variable length data is distributed so as to make the length of the variable length data equal to the above-mentioned block, and the error correction code is added by a block in which the data length has become equal, so as to carry out recording.
However, in a VCR, it is an effective means to make the tape winding angle small in searching so as to alleviate the burden of the mechanical system during high speed searching, and the construction of the foregoing reference involves a problem in that, as the fixed length data is recorded at equal intervals on the recording track in the recording medium, when the tape winding angle is reduced a recorded portion of the fixed length data does not come into contact with the reproducing head, which causes an extreme deterioration of the search image quality. Furthermore, in the case of reproducing the signal by using plural heads and improving the amount of data to be reproduced per specified time during high speed searching (hereinafter referred to as the data acquisition rate), a new reproducing circuit is required for processing in parallel with the reproducing circuit used during normal reproduction.